The present invention relates to a particularly advantageous process for preparing aryl mercaptans by hydrogenation of diaryl sulphides.
Aryl mercaptans are important intermediates for preparing pharmaceutically and agrochemically active compounds (see, for example, EP-A 100 172, BE-A 887 423, US-A 3 912 757 and WO 96/25936).
A useful process for preparing aryl mercaptans consists in the reaction sequence of the reduction of arylsulphonyl chlorides with sodium sulphite to arylsulphinic acid salts, further reduction with sulphuric acid to the diaryl disulphide and finally cleavage of the disulphide with sodium borohydride (DE-A 44 20 777). Unsatisfactory here is the cleavage of the disulphide with sodium borohydride which does proceed in good yield, but which requires sodium borohydride, a reagent which is difficult to prepare and thus expensive.
Catalytic hydrogenating cleavages of diaryl disulphides have also been described; however, they always proceed at high temperatures and pressures or require other disadvantageous measures. Thus, according to J. Org. Chem. 24, 1598 (1959), the reaction is carried out in toluene using an MoS2-aluminium oxide catalyst at 140xc2x0 C. and 124 bar. DE-A 17 68 421 describes this hydrogenation with palladium contacts and Raney cobalt at 160 to 200xc2x0 C. and 150 bar. According to JP-A (Japanese Published Specification) 60/199 871, the hydrogenation is carried out using Raney nickel in a heterogeneous solvent system comprising two phases, i.e. an aqueous/alkaline and a water-insoluble organic phase.
If these processes are carried out at lower pressures and lower temperatures, conversion is incomplete (see Comparative Examples 1 and 2).
We have now found a process for preparing aryl mercaptans by catalytic hydrogenation of diaryl disulphides which is characterized in that the hydrogenation is carried out in a basic alcoholic medium.
According to the process according to the invention, it is possible to prepare, for example, aryl mercaptans of the formula (I) 
in which
R and Rxe2x80x2 independently of one another each represent hydrogen, C1-C6-alkyl, C1-C6-alkoxy or halogen,
from the corresponding diaryl disulphides of the formula (II) 
xe2x80x83in which
R and Rxe2x80x2 are as defined in formula (I).
In the formulae (I) and (II), R and Rxe2x80x2 independently of one another preferably represent hydrogen, methyl, ethyl, methoxy, ethoxy, fluorine or chlorine. Particularly preferably, R and Rxe2x80x2 represent hydrogen.
The diaryl disulphides required as starting materials, in particular those of the formula (II), can be obtained by various routes, for example according to DE-A 44 20 777. Of particular interest here is a process in which S-(4-biphenyl)-sulphinic acid or salts thereof are reacted with an aqueous bisulphite solution at a pH of 2 to 7, thus giving the corresponding S-(4-biphenyl)-thiosulphuric acids or salts thereof, which are heated in the presence of a strong aqueous acid. This process is described in more detail in another patent application which was submitted simultaneously by the same applicant.
Suitable solvents for forming the alcoholic medium required according to the invention are, for example, aliphatic C1-C5-alcohols, which may be straight-chain or branched. They can be primary, secondary or tertiary alcohols. Preference is given to methanol, ethanol and isopropanol. The alcoholic medium may optionally also comprise water, for example up to 20% by weight, preferably up to 10% by weight. It is therefore, for example, not necessary to employ the diaryl disulphide in dry form. It can be used just as well in the form of the water-moist product obtained in its preparation. It is also possible to use mixtures of different alcohols as alcoholic medium.
Based on 1 mol of diaryl disulphide, it is possible to employ, for example, 200 to 10,000 ml of alcohol. This amount is preferably 500 to 5000 ml.
Suitable agents for generating a basic character in the alcoholic medium are, for example, alkali metal hydroxides, alkali metal carbonates or alkali metal alkoxides, in particular alkali metal C1-C4-alkyl alkoxides or the corresponding alkaline earth metal compounds. Preference is given to alkali metal hydroxides, in particular to sodium hydroxide and potassium hydroxide.
Based on 1 mol of diaryl disulphide, it is possible to employ, for example, 0.5 to 5 equivalents of base. This amount is preferably 1.5 to 2.5 equivalents.
Suitable catalysts for the hydrogenation according to the invention are, for example, metals of the VIII transition group of the PTE, in particular nickel and palladium. Such metals can be used as such or as metal compounds. Metals or metal compounds can optionally be arranged on carrier materials such as activated carbon, aluminium oxide, alkaline earth metal carbonate or alkaline earth metal sulphate. The catalysts may be doped with other metals or metal compounds, for example those of the 4th or 8th transition group of the PTE. They can also be present in the form of skeleton catalysts, for example in Raney form. Preferred catalysts are palladium on carbon, palladium black, Raney nickel and Raney nickel which is doped with cobalt and/or iron. Particular preference is given to Raney nickel.
The catalytic hydrogenation according to the invention can be carried out, for example, at temperatures of 20 to 200xc2x0 C. and pressures of up to 50 bar. Preference is given to temperatures of 40 to 150xc2x0 C. and pressures of up to 25 bar, in particular to temperatures of 60 to 120xc2x0 C. and pressures of up to 15 bar.
Work-up of the reaction mixture which is present after the hydrogenation according to the invention can be carried out, for example, by cooling, for example to 20 to 65xc2x0 C., separating off the catalyst, for example by filtration, treating the resulting alcoholic solution of the aryl mercaptan which has been prepared with acid and, if appropriate, water and then filtering off and drying the precipitated product.
Using the process according to the invention, it is possible to obtain aryl mercaptans, in particular diaryl mercaptans of the formula (I), in good yields and purities.
The process according to the invention has surprising advantages. Thus, it does not require any reagents which are difficult to prepare. It can be carried out at lower temperatures and in particular at lower pressures than other processes, which means less expenditure on apparatus and handling. Finally, a complicated procedure with two liquid phases present side by side is avoided. Particularly surprising here is the fact that, from Houben-Weyl, Methoden der organischen Chemie, Volume IX, p. 77 (1955), it is known that in basic medium disulphides undergo disproportionation into sulphinates and thiols. Hitherto, it was attempted to suppress this undesirable reaction by using water-immiscible solvents. Surprisingly, it was found that this is not necessary.
The invention is further described in the following illustrative examples in which all parts and percentages are by weight unless otherwise indicated.